The present invention relates to processes for etching semiconductor wafers.
Reactive ion etching processes are used in the fabrication of semiconductor and other devices with submicron sized features, such as integrated circuit chips. These processes are used to selectively etch a substrate, where portions of the substrate are protected by a patterned etch-resistant "resist" material, such as photoresist or oxide-hardmask. The resist protected portions form "features" on the substrate which become part of the integrated circuit being processed. Etching is effected by introducing a process gas into a chamber and generating a plasma in the chamber, to create an etch gas from the process gas. The etch gas etches the substrate to create a volatile etch byproduct compound, which is then removed from the chamber.
Typically, the process gas comprises Cl.sub.2, BCl.sub.3 and optionally N.sub.2. A problem with this process gas is that relatively thick residues or deposits form on the chamber walls and on the resist material. Some of these deposits form because the etch gas chemically reacts with the resist material. The deposit on the chamber walls can flake off to form particles that contaminate the wafers. Further, the formation of excessive amounts of residue on the resist can undesirably increase the area protected by the resist material, rendering the processed wafer unsuitable for use.
A deposited layer can also form on the sidewalls of the freshly etched channels in the substrate. This deposit serves as a "passivating" layer which hinders continued etching, thereby preventing "isotropic" etching or undercutting. Isotropic etching occurs when etching proceeds horizontally below the resist layer, instead of vertically through the uncoated portions, resulting in the lower portions of the features being inwardly sloped. Although vertical "anisotropic" etching is desirable, an excessive deposit of passivating layer on the sidewalls is difficult to clean. Thus, it is desirable to have an etching process that either produces less build-up of deposit, or which reduces build-up by etching the deposit layers during the etch process.
Furthermore, typical reactive-ion etching systems result in high profile microloading. High profile microloading causes the cross-sectional profile of the features formed in the substrate to vary as a function of the spacing between the features. It is desirable to have an etching process which provides features with a uniform cross-section regardless of the spacing between the features.
It is also desirable to obtain high etch rates for process efficiency and to obtain a high etching selectivity ratio so that the rate of etching of the resist layer is substantially lower than the rate of etching of the substrate.
In plasma etch processes, it is also desirable for the process gases to have a low ionization potential so that less energy is required to ionize the gases. It is also advantageous that the process gases have a wide range of excitation energies so that energy transfer reactions, which promote etching efficacy, can occur between the various gaseous species. Also, gases with low corrosivity are preferred so that the corrosive effect of the etch gas on the processing apparatus is minimized.
Accordingly, there is a need for a process for selectively etching semiconductor substrates which minimizes formation of deposits on the chamber walls, which does not form excessive deposits on the sidewalls of the etched channels, which provides substantially anisotropic etching, which does not result in formation of oversized features, and which reduces profile microloading. It is also desirable to obtain high etch rates and a high substrate-to-resist etch selectivity ratio. Also, it is preferred that the process gases have low corrosivity, low ionization potential, and a broad range of excitation energies.